1. Field of the Invention
The present invention relates to new tilt lens systems and image pickup apparatuses. More particularly, the present invention relates to a tilt lens system, and an image pickup apparatus having the tilt lens system, for a video camera, a digital camera, or a digital camera or a film camera of single-lens reflex type. The tilt lens system has a tilt function, capable of focusing on not only an object arranged in parallel to an image pickup surface, but also an object arranged in a tilted manner with respect to the image pickup surface, without degrading an autofocus function and an exposure measurement function.
2. Description of the Related Art
In a typical image pickup apparatus, the optical axis of a lens is arranged perpendicularly to an image pickup surface, which is an image pickup device or a film, at the center thereof. Hence, for example, when a high building is photographed at a low angle, the resultant perspective causes the top of the building to appear smaller than its base. In addition, it is difficult to focus on the entire object plane which is tilted with respect to the image pickup surface. An aperture of a diaphragm is decreased to increase the depth of field, to photograph a picture as in focus. To reduce the problem in photographing, the optical axis of the lens is moved in parallel to the image pickup surface, or tilted with respect to the image pickup surface. Such movements are called perspective control.
To compensate the perspective caused by the low angle shot, an object, the principal plane of the lens, and the image pickup surface are arranged and held to be parallel to each other, and a photographing range is thus determined. This operation, called flaming, is performed by shifting the position at which the optical axis meets the image pickup surface. This kind of perspective control operation is usually called shift, rise, fall, and the like, depending on the shift direction of the operation, however, in this specification, the operation in any shift direction is called shift.
To focus on a plane tilted with respect to the image pickup surface, the principal plane of the lens and the image pickup surface are moved to not be parallel to each other. This kind of perspective control operation is usually called swing, tilt, and the like, depending on the tilt direction of the operation; however, in this specification, the operation in any tilt direction is called tilt.
Examples of the image pickup apparatuses capable of performing the perspective control may be cameras called view cameras and folding dark boxes in which a lens board and an image pickup surface are connected through a concertina-like cloth, so that the positional relationship therebetween can be desirably controlled. However, in this type of camera, a perspective control operation and focusing have to be performed while a user is viewing an inverted image projected on a focusing screen. Such cameras are extremely difficult to use, and therefore, the cameras are considered as special cameras.
A single-lens reflex camera is a popular type of camera. As its interchangeable lens, there is provided one having shift and tilt functions. This provides a more user-friendly perspective control operation in comparison with a view camera. However, if the perspective control operation is performed with this lens, an area outside an image height of a normal screen size may be used. In lens design, it is desired to cover a larger image circle than that of the normal interchangeable lens, with an image quality equivalent to that of the normal interchangeable lens. Japanese Unexamined Patent Application Publication No. 2000-292689 discloses an example of such an interchangeable lens.
In recent years, most of the image pickup apparatuses are digital cameras using image pickup devices instead of films. Even if films are used for photographing, the pictures are converted into digital information with scanners, and output to various media. To output the digital image information as a printout, a printing original, a computer image, or the like, the digital image information is frequently processed using an image editing software called a photo retouch software. The photo retouch software has various functions, such as adjustment of color, brightness, contrast, and sharpness; trimming; image synthesis; etc. One of the various functions may be a perspective compensation function. In particular, in a case where a normal camera is used for photographing and a building is photographed by low angle shot, thereby the top of the building being smaller than its base, the image is deformed to a trapezoid, and then is trimmed to a rectangle, so that the width of the top of the building is increased by digital signal processing. Accordingly, an effect of photographing with shifting can be provided for the image. Since the image is deformed to the trapezoid, the resolution of the extended upper portion of the screen may be decreased. However, data from the single-lens reflex digital camera, or data read from the film using the scanner with a high pixel pitch originally has a sufficient resolution. Hence, a certain decrease in the resolution does not have to be considered. Since the use of such a photo retouch software has become popular in recent years, a shift operation, which is difficult to use during photographing, is no longer necessary.
Also, one of effects of the tilt operation provides a photographing method, in which, when a landscape at infinity is photographed, a part of the screen is intentionally focused and the residual part is defocused, so as to make an actual landscape to be photographed like a diorama at close range. This effect may also be provided by the photo retouch software. In particular, an image in focus entirely is used, the image except for a part of the image is intentionally defocused, and accordingly, the image has an effect like being defocused by tilting.
Therefore, the effect not provided by the photo retouch software, and provided only by the perspective control lens is an effect satisfying the Scheimpflug rule, in which, when an object plane, the principal plane of the lens, and the image pickup surface are arranged to intersect with each other at a ridge, a tilted object plane is in focus.
Japanese Unexamined Patent Application Publication No. 4-243212 and No. 4-243207 disclose related arts capable of performing the tilt operation.